This invention relates generally to permanent magnets and, more particularly, to configurations of permanent magnets used to produce a magnetic field having a required strength and flux alignment at a specified situs.
The use of permanent magnets to produce and maintain magnetic fields of predetermined characteristics has found wide-spread application throughout varied areas of technology. As one example, the use of permanent magnets in loudspeakers makes possible the accurate reproduction of sound and music.
Permanent magnets offer numerous advantages over electromagnets, the most prominent of which is the creation of a magnetic field without electrical energy, or an external power source. This is an important consideration in designing portable apparatus, and also results in constructions requiring little or no maintenance beyond initial assembly.
The present invention utilizes particular arrangements and configurations of permanent magnets to create strong, uniform magnetic fields. Although the uses to which the present invention may be put are described herein as principally in connection with medical diagnostic equipment, it should be understood that the invention may be utilized wherever a magnetic field is required.
The present application also discloses means and methods for adjusting and enhancing the magnetic fields produced by permanent magnets.
As described in the above-mentioned patent applications, the use of magnetic fields makes possible the non-invasive testing of human body systems to detect the presence of certain substances. In particular, permanent magnets are used to create a first, or biasing magnetic field to align initially randomly oriented .sup.1 H protons present in the nuclei of the substance in the sample being tested. Thereafter, a second energy field is applied to increase the energy level of said nuclei. When the second energy field is allowed to collapse, the nuclei return to their original, unaligned state, releasing energy which is detected and analyzed in the form of an image or spectrum. Certain of such spectra have been found to be characteristic of individual substances, and this technique of nuclear magnetic resonance (NMR) spectroscopy may be used to establish the presence and identity of such substances and the concentrations in which such substances are present.
In utilizing NMR diagnostic techniques, creating and maintaining the primary or first magnetic field is of critical importance. The ability of the field to resolve the signal that occurs when the secondary field collapses can be characterized as the ratio of overall field strength to the field gradient present across the portion of the primary field within which the test sample is held. As an example, if the primary field strength is 10,000 gauss, and the change in field strength across the test region is 0.01 gauss, the field is said to have a resolution of 0.01/10,000, or one part per million (ppm).
Heretofore, the use of NMR has called for the construction of large, expensive machinery to produce and maintain the magnetic fields necessary for such testing. To make NMR equipment compact and portable, the primary magnetic field which is uniform in strength and direction, and which has a relatively dense magnetic flux, must be created by a relatively light and compact arrangement of magnets. Visually, such a field may be defined by the lines of flux which indicate not only the strength of the magnetic field, but its uniformity and orientation as well.
Ideally, the lines of magnetic flux should be parallel or nearly parallel and uniform in strength throughout i.e., that portion of the magnetic field used for diagnostic purposes (identified herein as the "test region"). It is efficacious to produce the first or primary magnetic field with permanent magnets rather than electromagnets.
The use of magnets and magnetic energy to diagnose and treat biological disfunctions is well known. As an example, in French Patent No. 2,562,785 (Jeandey, et al.) a permanent magnet system for NMR imaging medical diagnostics uses pole pieces separated by stacked permanent magnets to form an open examination area with the pole pieces "bridging" both stacks of magnets. Jeanday, et al. also teach the use of electromagnetic coils to adjust the resulting magnetic field.
Japanese Patent No. 56-14145 (Nippon Denshi K.K.) teaches an arrangement of permanent magnets held within a cylinder. A spacer is placed within the cylinder and sandwiched about the spacer are a pair of cylindrical pole pieces. The entire assembly is held together by placing magnets outside the pole pieces (separated from the pole pieces by a buffer) and utilizing the attraction of the magnets for each other to hold the entire assembly in place. Nippon Denshi also teaches the use of pole pieces having raised central portions, that is, flat faces which extend into the air gap between the pole pieces and from which the operative flux emanates. Nippon Denshi fails to teach any use of auxiliary magnets in combination with the principal magnets.
U.S. Pat. No. 4,635,643 (Brown) teaches the use of NMR equipment to perform in vivo measurement of the mineral content of bone. Brown, however, teaches no arrangement of permanent magnets in constructing a test chamber for NMR use.
In U.S. Pat. No. 4,134,395 (Davis) the patentee teaches the use of a permanent magnet to detect diseased body parts by observing the effect of a magnetic field on the muscles of the legs. Davis also describes the physical characteristics of a bar magnet showing the extent and shape of the magnetic field produced by such a magnet.
U.S. Pat. No. 3,467,076 (Frisch, et al.) discloses a magnet arrangement used to produce a field of high magnetic flux within which the effect of magnetic energy on living things may be observed. Frisch, et al., use a centrally-located electromagnet sandwiched between ferromagnetic pole pieces which extend beyond the edges of electromagnet to form an air gap within which the magnetic flux is produced.
In U.S. Pat. No. 3,358,676 (Frei, et al.), a method of treatment is taught which requires the use of an extremely large and cumbersome magnet structure. The size and complexity of magnets required to utilize the phenomenon of NMR in making diagnoses is also exemplified in an article appearing in the December, 1977 issue of Popular Science magazine, entitled "Damadian's Supermagnet". The author discusses the use of NMR to detect cancer cells: the size and strength of the magnetic field required to perform this diagnostic technique, and the size and complexity of the magnet used to produce such a field are well described.